1. Field of the Invention
The present invention relates to non-destructive testing, and more particularly, to an eddy current means and method for detecting cracks in metal components such as the material around fastener holes.
In the aircraft industry, for example, it is known that fatigue cracks develop during the lifetime of components which repeatedly go through cycles of stress in normal operation. One particular location of such cracks which presents a serious problem because of their invisibility is in the holes around fasteners such as rivets, screws, and the like, particularly in the second layer under the fastener. For instance, in the large C-5A U.S. Air Force transport, there are approximately 17,000 critical fastener locations per aircraft.
2. Description of the Prior Art
Eddy current methods have been used to find these cracks when they have become quite large, by removing the fastener, and rotating an eddy current probe in the hole to determine if a crack is present. If no crack exists, a new fastener is installed. If a crack is found, the hole is reworked by enlarging it and then a new, larger size, fastener is installed, or a threaded insert may be installed in some cases.
Ultrasonic inspection methods have also been tried, without fastener removal, but this method has been unable to penetrate to inspect the second or lower layer of material under the fastener.
U.S. Pat. No. 3,855,530 to Fuji et al discloses the use of two coils wound on respectively perpendicular cores crossing at the center to provide two pairs of magnetic poles intersecting each other when the coils are energized on alternate half-cycles of a power supply voltage. The device can indicate flaws in a plane magnetic member by the positioning of magnetic powder spread on the member surface. This device does not actually produce eddy currents for the purposes used in an eddy current testing method.
U.S. Pat. No. 3,609,531 to Forster describes an eddy current probe that is effectively mechanically scanned along pipe for detecting cracks.
U.S. Pat. No. 4,155,455 to Spierer et al similarly makes use of a conventional eddy current test probe positioned adjacent to small magnetic material parts which are physically rotated while being tested for defects.
U.S. Pat. No. 2,558,485 to Gow shows a coaxial cable testing system wherein a three-phase rotating magnetic field is used to check the position of the center conductor of the cable when placed between the tips of the salient poles. No eddy current is generated, nor is there any crack detection.
In none of these patents is there found a teaching of detecting a crack particularly around the hole of an installed fastener, or in the second layer of material held together under the head of a fastener, or of scanning a test piece with an electrically rotating field to search for defects.
While it is possible that more pertinent prior art exists, Applicant's search is believed to have been conducted with a conscientious effort to locate and evaluate the most relevant art available at the time, but this statement is not to be construed as a representation that no more pertinent art exists.
Thus it is an object of this invention to provide a means and method of detecting small fatigue cracks in the second layer of sheet metal structure, for example.
Because of the very great number of fastener locations to be periodically inspected in an aircraft, another object of the present invention is to provide a testing method which is fast.
Still another object is to provide an eddy current inspection system which is inherently less sensitive to lift-off effects than prior known systems, i.e., minor and unavoidable variations in the position of the testing probe on the article or part being inspected. Such effects cause errors in the output indications from conventional mechanically scanned probes.